Tin plate treatment



United States Patent 3,138,548 TIN PLATE TREATMENT Coy S. Ham, Munster, and Jack E. Joyce, Chesterton, Ind., assignors to Inland Steel Company, Chicago, 111-, a corporation of Delaware No Drawing. Filed Apr. 11, 1961, Ser. No. 102,143 7 Claims. (Cl. 204-32) The present invention relates generally to a method of treating tin plate to improve the surface properties of the tin plate and to the treated tin plate produced. More particularly, the present invention relates to an improved electrolytic tin plate and to a method of treating electrolytic tin plate to improve its lacquer adherent properties.

A large percentage of the tin plate used for cans and other containers, particularly containers for foods and beverages, is produced commercially by the electroplating of steel strips. In producing electrolytic tin plate commercially, a continuous strip of steel, such as blackplate, is progressively moved through a long electroplating line. As the strip moves through the line, usually at speeds of about one thousand feet per minute, it is cleaned, pickled, scrubbed, and then electroplated with a coating of tin. After the strip has been plated, usually on both surfaces, it is washed to remove any residual electrolyte and the tin coating is flow brightened to convert the tin from a condition in which it presents a matte surface to a condition in which it presents a bright mirror-like surface. The flow brightening is effected by melting and solidifying the tin coating. The flow brightened tin plate strip is then usually treated chemically and wound into large coils for storage or shipping. Subsequently, the coil is trimmed, cut and thereafter processed to form the containers.

When tin plate is used to form containers for certain materials, such as foods, it is common practice to coat the tin plate forming the interior surface of the container with a protective organic film, such as a phenolic lacquer. Where the lacquer is not firmly held by the tin plate and separates therefrom, the foods will often react with the tin plate. It is thus of great importance for the tin plate to have good lacquer adherent properties.

It has heretofore been observed that during passage of the steel strip through the electrolytic plating line and in the tin fusing furnace, objectionable oxides or other surface products are formed, particularly when the tin plating is heated above its melting point without benefit of a non-oxidizing atmosphere to provide a smooth surface having a bright lustrous finish. Since phenolic lacquers are very sensitive and will not adhere satisfactorily to a tin plate surface which has a significant amount of oxide or other objectionable surface contamination, it is essential that the tin plate after fusing must be treated to remove these objectionable surface products. And, since lacquering generally does not take place immediately after removal of these surface products, the tin plate must also be passivated to prevent additional oxides or the like products forming on the tin plate surface during storage.

A great many chemical and electrochemical filming treatments have been developed heretofore which have as their objective the removal of contaminants and retardation of the growth of objectionable oxide films, thereby reendering the tin plate surface resistant to oxidation and improving the lacquer adherent properties thereof. The more successful of these treatments all use a soluble chromium compound in at least one of the treating solutions which may or may not include an electrolytic step. None of the prior art methods of treating tin plate, however, have been able to impart entirely satisfactory lacquer adherent properties to electrolytically produced tin 3,138,548 Patented June 23, 1964 plate having considerable surface oxide formation, particularly tin plate produced on an alkaline electrolytic tin line after the tin surface has been fused to provide the desired smooth bright surface.

It is therefore an object of the present invention to provide a method of improving the lacquer adherent properties of tin plate.

It is also an object of the present invention to provide a method of treating tin plate to improve the lacquer-adherent characteristics thereof while also imparting good resistance to discoloration.

' It is a further object of the present invention to provide an improved tin plate product characterized by improved lacquer adherent properties.

Other objects of the present invention will be apparent to those skilled in the art from the accompanying detailed description and claims to follow.

It has now been discovered that tin plate having very substantially improved lacquer adherence can be produced by subjecting a tin plate surface after fusion thereof to treatment by a combination of steps comprising:

(1) Cathodic treatment in an alkaline electrolyte (2) Water rinse (3) Dilute acid dip to bring the surface of the plate to about pH 4-6 (4) Cathodic chromate treatment in an acidic electrolyte having a pH of about pH 4-6.

The method of treating tin plate comprising the foregoing novel combination of steps is particularly suited for use on tin plate having a relatively large amount of oxide on the surface, such as produced by a gas fusion furnace or by electrolytic action, which has not heretofore had entirely satisfactory enamel or lacquer adherent properties. It should be understood, however, that the process of the present invention is not restricted to treating tin plate produced on an alkaline or an acidic electrolytic tin plate line or by any particular method.

Thus, whereas an alkaline cathodic treatment followed by a water rinse and chromic acid dip has produced only fair results when applied to fused tin plate produced on an alkaline electrolytic tin line and a somewhat poorer lacquer adherent surface than produced by a cathodic dichromate treatment of fused tin plate produced on an acid electrolytic tin line, the herein disclosed novel combination of steps produces a tin plate having better lacquer adherence than either of the foregoing standard treatments.

In treating tin plate by the process of the present invention, it is necessary to first remove surface contaminants and oxides produced during or after tin plating and fusion but prior to passivating. The surface contaminants and oxides on the tin plate are substantially removed in the first step of the instant process by cathod ically treating the tin plate in a relatively dilute alkaline electrolyte, such as a sodium carbonate solution. Thus, for example, a sodium carbonate solution having a concentration between about 1% and 10% by weight, and preferably between 3 and 5% by weight, when correlated with temperature, time of treatment, and current density to avoid reacting with the tin, has produced good oxide removal and cleaning results. However, a solution of any equivalent alkali metal compound can be used in equivalent concentrations and under equivalent conditions. Other compounds or mixtures thereof which can be used in place of sodium carbonate are sodium hydroxide, trisodium phosphate, potassium carbonate, potassium hydroxide, sodium bicarbonate, mono-sodium hydrogen phosphate, di-sodium hydrogen phosphate, and potassium bicarbonate. The current density in the electrolyte can be maintained between about 20 and 60 coulombs/ft preferably at about 30-40 coulombs/K while the temperature of the electrolyte is between about 100 to 200 F., and preferably at about 175-185 F.

After being briefly treated cathodically in the alkaline electrolyte, the tin plate is removed, washed with water, and immersed in an acidic conditioning bath prior to being subjected to an acidic cathodic passivating treatment. Thus, while heretofore it has been accepted practice to passivate the tin plate by immersing the plate in a chromic acid or equivalent solution after cathodic alkaline cleaning and a water rinse, it has now been discovered that a markedly improved lacquer adherent tin surface is produced if the tin plate surface is acid conditioned prior to being treated cathodically in an acidic hexavalent chromium electrolyte or equivalent solution.

The conditioning treatment to which the cleaned tin plate is subjected in the instant process comprises immersing the tin plate in a dilute acidic solution having a pH between about pH 4 and 6 which corresponds substantially to the pH of the subsequently applied cathodic electrolyte treatment step. The acid treatment bath can comprise an aqueous solution of either a water soluble organic or inorganic acid. Among the organic acids which can be used are acetic acid, formic acid, citric acid, oxalic and tartaric acid with acetic acid being preferred. Inorganic acids which can be used include hydrochloric acid and hydrofluoric acid. The acid conditioning solution is preferably prepared by using demineralized or soft water to avoid extraneous contamination. With the aqueous conditioning solution acidified with an organic acid, such as acetic acid, to a pH of between pH 4 and 6, the conditioning solution is maintained between about 100 to 200 F. and preferably at about ISO-170 F., the latter temperature being sufficient to heat the tin plate to substantially the temperature of the electrolyte in the subsequent passivating treatment. The tin plate is allowed to remain in the acid bath for a period of only about 1-3 seconds, after which it is withdrawn and subjected directly to a cathodic passivating electrolyte treatment. It should be evident that the treatment in the acidic bath, while effecting a conditioning of the tin plate surface, will effectively neutralize the residual alkaline material from the alkaline cleaning step which remains on the surface of the tin plate.

The cathodic passivating treatment which is applied to the conditioned tin plate in the instant process comprises making the tin plate the cathode in a heated aqueous electrolyte containing hexavalent chromium ions with the electrolyte having a pH of about pH 4-6 while passing an electric current through the electrolyte. Whereas sodium dichromate is the preferred hexavalent chromium compound for the cathodic passivating treatment, any other hexavalent chromium compound or compound which is converted to a hexavalent chromium compound can be used, including sodium chromate, potassium chromate, potassium dichromate, and chromic acid. The hexavalent chromium ion concentration in the electrolyte can range between a concentration of hexavalent chromium ion equivalent to about 1% to about 10% by weight of sodium dichromate, and preferably equivalent to about 2-3% by weight sodium dichromate, so that the pH of the electrolyte does not depart appreciably from the range of about pH 46. The temperature of the hexavalent chromium containing electrolyte bath can be maintained between about 100 F. and 200 F. but preferably is held between about 120 F. and 140 F. The current density which is used can range between about to about 60 coulombs per ft. but is preferably maintained at about -40 coulombs per ft. It should be understood, however, that the limits specified for each of the operating conditions described herein are interdependent and also depend on the manner and period of time during which the tin plate is allowed to remain in contact with the electrolyte. Thus, in the foregoing discussion the ranges 4.- specified are applicable generally to a process in which an endless strip of tin plate is passed continuously through an electrolyte bath at a rate of about 500 to 1200 feet per minute.

Since the pH of the acid conditioning bath is substantially the same as the pH of the passivating electrolyte there is no water rinse required between the conditioning and passivating steps. And, while some carry-over of the acid solution to the acidic cathodic passivating electrolyte takes place, the presence of an acid, such as the preferred acetic acid, in the hexavalent chromium passivating electrolyte does not impair the action of the passivating electrolyte and under some circumstances further improves the surface properties of the tin plate.

In order to further illustrate the present invention the following specific example is set forth without, however, limiting the invention to the particular conditions or reagents employed:

Tin plate produced electrolytically on an alkaline tin plating line in endless strip form and after flow-brightening in a gas fusion furnace is passed continuously at a rate of about 700 ft. per minute line speed through an alkaline electrolyte comprising about 4 percent by weight of sodium carbonate. The strip is made the cathode while immersed in the alkaline electrolyte and an electric current having a current density of about 35 coulombs per ft. is passed through the electrolyte and the strip while the electrolyte is maintained at a temperature of about 175-185 F. The strip is moved from the sodium carbonate electrolyte through a water wash and then passed between air jets which remove all the wash water from the surface of the strip. The strip next enters a nonelectrolytic acid conditioning bath comprising an aqueous acetic acid solution having a pH between 4 and 6 comprising between about .005 to .02 percent by volume glacial acetic acid. The acid solution is heated to a temperature of about ISO-170 F. Thereafter the strip is passed directly into a heated aqueous sodium dichromate electrolyte containing about 20-30 grams sodium dichromate per liter and having a pH of about pH 4-6. The electrolyte is maintained at a temperature of about 140 F. Upon entering the dichromate electrolyte, the strip is made the cathode and an electric current having a density of 30-40 coulombs per ft. is passed through the electrolyte and the strip. The strip is rinsed with water as it leaves the dichromate electrolyte and dried. The dried, passivated tin plate strip is preferably oiled or otherwise coated with a protective surface film prior to coiling or the strip may be trimmed and cut without coiling, if desired.

The tin plate produced in the foregoing manner exhibits a strong adherence for lacquers, including phenolic lacquers, and at the same time is highly resistant to discoloration.

It will be understood by those skilled in the art that the process of the present invention, as illustrated in the foregoing specific example, can be carried out with a combination of conventional equipment presently employed for alkaline cathodic cleaning, water washing, chromic acid dipping, and cathodic dichromate passivating treatments.

It should be understood that various modifications can be made in the individual steps of the process without departing from the present invention comprising the novel combination of steps herein described in order to adapt the invention for use with a given electroplating line or hot dip galvanizing line, whereby the herein described improved surfaee characteristics are imparted, particularly improved lacquer adherent properties, and also resistance to discoloration.

We claim:

1. A method of treating tin plate so as to provide improved lacquer adherent surface characteristics which comprises in combination the steps of making the tin plate the cathode while immersed in a hot aqueous alkaline cleaning electrolyte and passing an electric current through said electrolyte and tin plate to remove substantially all surface oxides and other contamination thereon without removing tin, washing said tin plate with water after it is withdrawn from said alkaline cleaning electrolyte to rinse away most of said alkaline cleaning electrolyte remaining on the surface thereof, thereafter applying to said tin plate after washing with water and before subjecting to electrolytic treatment with a chrominum containing solution an acid conditioning solution comprising a hot dilute aqueous acid solution having a pH between about pH 4 and 6, and directly passivating the acid conditioned tin plate surface by making said tin plate the cathode while immersing in a hot aqueous hexavalent chromium containing electrolyte having a pH between about pH 4 and 6 while passing an electric current through said chromium containing electrolyte and tin plate to provide a passive nonoxidizing surface on said tin plate; whereby a tin plate having substantially improved lacquer adherent surface characteristics is provided.

2. A method of treating tin plate so as to provide improved lacquer adherent surface characteristics thereof which comprises in combination the steps of making the tin plate the cathode While immersed in a hot aqueous alkaline cleaning electrolyte containing an alkali metal compound in an amount between about 1% to by weight and passing an electric current having a density between about 20 and 60 coulombs per ft. through said electrolyte and tin plate to remove substantially all surface oxides and other contamination thereon without removing tin, washing said tin plate with water after it is withdrawn from said alkaline cleaning electrolyte to rinse away most of said alkaline cleaning electrolyte remaining on the surface thereof, thereafter applying to said tin plate after washing with water and before subjecting to electrolytic treatment with a chromium containing solution an acid conditioning solution comprising a hot dilute aqueous acid solution having a pH between about pH 4 and 6, and directly passivating the acid conditioned tin plate surface by immersing said tin plate in a hot acidic aqueous hexavalent chromium containing electrolyte having a hexavalent chromium ion concentration equivalent to between about 1% and 10% by weight of sodium dichromate, said electrolyte having a pH of between about pH 4-6, and while said tin plate is the cathode in said chromium containing electrolyte and simultaneously passing an electric current having a current density of about 20 to 60 coulombs per ft. through said chromium containing electrolyte and tin plate to provide a passive nonoxidizing surface on said tin plate; whereby a tin plate having substantially improved lacquer adherent surface characteristics is provided.

3. A method of treating tin plate so as to provide improved lacquer adherent surface characteristics thereof which comprises in combination the steps of making the tin plate the cathode while immersed in a hot aqueous alkaline cleaning electrolyte containing an alkali metal compound in an amount between about 1% and 10% by weight and having a temperature between about 100 F. and 200 F. and passing an electric current having a density between about 20 and 60 coulombs per ft. through said electrolyte and tin plate to remove substantially all surface oxides and other contamination thereon without removing tin, said alkaline compound being selected from the group consisting of sodium carbonate, sodium hydroxide, tri-sodium phosphate, potassium carbonate, potassium hydroxide, sodium bicarbonate, potassium bicarbonate, mono-sodium hydrogen phosphate, disodium hydrogen phosphate, washing said tin plate with water after it is withdrawn from said alkaline cleaning electrolyte to rinse away most of said alkaline cleaning electrolyte remaining on the surface thereof, applying to said washed tin plate a conditioning solution comprising a hot dilute aqueous acid solution having a pH be tween about pH 4 and 6 and a temperature between about F. and 200 F., said acid being selected from the group consisting of acetic acid, formic acid, citric acid, oxalic acid, tartaric acid, hydrochloric acid, and hydrofluoric acid, and directly passivating the acid conditioned tin plate surface by immersing said tin plate in a hot acidic aqueous hexavalent chromium compound containing electrolyte having a hexavalent chromium ion concentration equivalent to between about 1% and 10% by weight of sodium dichromate, said chromium compound being selected from the group consisting of sodium dichromate, potassium dichromate, sodium chromate, potassium chromate, and chromic acid and said chromium containing electrolyte having a pH of between about pH 4 and 6 and a temperature between about 100 F. and 200 F., and while the said tin plate is the cathode in said chromium containing electrolyte passing an electric current having a current density of about 20 to 60 coulombs per ft. through said chromium containing electrolyte and tin plate to provide a passive non-oxidizing surface on said tin plate; whereby a tin plate having substantially improved lacquer adherent surface charcteristics is provided.

4. A method of improving the lacquer adherent properties of an electroplated tin plate strip after the tin coating thereof has been fused to provide a smooth bright surface which comprises; continuously passing the said tin plate strip through a series of treating steps comprising cleaning in an alkaline cleaning electrolyte comprising a hot aqueous solution of an alkali metal compound in which the concentration of the alkali metal compound is between about 1% and 10% by weight and having a temperature between about 100 F. and 200 F. and while the tin plate strip is the cathode in said alkaline cleaning electrolyte passing an electric current having a density of between about 20 and 60 conlombs per ft. through said electrolyte and said tin plate to remove surface contamination and oxides from the said strip without removing tin, Washing the said strip with water after it leaves said alkaline cleaning electrolyte to rinse away most of said alkaline cleaning electrolyte remaining on the surface thereof, thereafter immersing said strip for a period of about 1 to 3 seconds in an acid conditioning solution comprising a dilute aqueous solution of an acid after Washing with water and before subjecting to electrolytic treatment with a chromium containing solution, said dilute aqueous solution of an acid having a pH between about 4 and 6 and a temperature between about 100 F. and 200 F., and directly passivating the acid conditioned strip by immersing said strip in an acidic electrolyte comprising an aqueous solution of a hexavalent chromium compound in which the said chromium compound provides a hexavalent chromium ion concentration equivalent to the chromium ion concentration of between about 1% and 10% by weight sodium dichromate, said acidic electrolyte having a pH between about 4 and 6 and a temperature between about 100 F. and 200 F., and While said strip is the cathode in said chromium containing electrolyte passing an electric current having a density of between about 20 and 60 conlombs per ft. through said chromium containing electrolyte and said strip to provide a passive non-oxidizing surface on said strip; whereby a tin plate strip having good adherence for lacquers is provided.

5. A method of improving the lacquer adherent properties of an electroplated tin plate strip after the tin coating thereof has been fused to provide a smooth bright surface which comprises; passing the said tin plate strip through an alkaline cleaning electrolyte comprising a hot aqueous solution of an alkali metal compound in which the concentration of the alkali metal compound is between about 1% and 10% by weight and having a temperature between about 100 F. and 200 F., and while the tin plate strip is the cathode in said alkaline cleaning electrolyte passing an electric current through said alka- U line cleaning electrolyte and said tin plate at a current density of between about 20 and 60 coulombs per ft. to remove surface contamination and oxides from the said strip without removing tin, said alkaline compound being selected from the group consisting of sodium carbonate, sodium hydroxide, tri-sodium phosphate, potassium carbonate, potassium hydroxide, sodium bicarbonate, potassium bicarbonate, mono-sodium hydrogen phosphate, disodium hydrogen phosphate, washing said strip with water after it leaves said alkaline cleaning electrolyte to rinse away most of said alkaline cleaning electrolyte remaining on the surface thereof, applying to said washed strip a conditioning solution comprising a dilute aqueous solution of an acid having a pH between about 4 and 6 and a temperature between about 100 F. and 200 F., said acid being selected from the group consisting of acetic acid, formic acid, citric acid, oxalic acid, tartaric acid, hydrochloric acid, and hydrofluoric acid, and directly passivating the acid conditioned strip by immersing said strip in an acidic electrolyte comprising an aqueous solution of a hexavalent chromium compound in which the said chromium compound provides a hexavalent chromium ion concentration equivalent to the chromium ion concentration of between about 1% and 10% by weight sodium dichromate, said chromium compound being selected from the group consisting of sodium dichromate, potassium dichromate, sodium chromate, potassium chromate, and chromic acid, said acidic electrolytic having a pH between about 4 and 6 and a temperature between about 100 F. and 200 F., and while said strip is the cathode in said chromium containing electrolyte passing an electric current having a density of between about 20 and 60 coulombs per ft. through said chromium containing electrolyte and said strip to provide a passive nonoxidizing surface on said strip; whereby a tin plate strip having good adherence for lacquer is provided.

6. A method of improving the lacquer adherent properties of an alkaline electroplated tin plate strip after the tin coating thereof has been fused to provide a smooth bright surface comprising; passing said tin plate strip moving at a rate of about 700 feet per minute through an alkaline cleaning electrolyte comprising a hot aqueous solution of sodium carbonate in which the concentration of said carbonate is about 4 percent by weight and having a temperature between about l185 F., and while the tin plate strip is the cathode in said alkaline cleaning electrolyte passing an electric current through said electrolyte and said tin plate at a current density of about 35 coulombs per ft. to remove surface contamination and oxides from the said strip without removing tin, washing said strip with water after it leaves said electrolyte to rinse away most of said electrolyte remaining on the surface thereof, applying to said washed strip a conditioning solution comprising a dilute aqueous solution of between about .005 and .02 percent by volume glacial acetic acid and having a pH between about 4 and 6 and a temperature between about F. and 200 F., and directly passivating the acid conditioned strip by immersing said strip in an acidic electrolyte comprising an aqeuous solution of about 20-30 grams sodium dichromate per liter and said acidic electrolyte having a pH of between about 4 and 6 and a temperature between about and F., and while said strip is the cathode in said acidic electrolyte passing an electric current having a density of between about 30 and 40 coulombs per ft. through said acidic electrolyte and said strip to provide a passive non-oxidizing surface on said strip; whereby a tin plate strip having good adherence for lacquer is provided.

7. A tin plate treated by the process of claim 6 having good lacquer adherent properties.

References Cited in the file of this patent UNITED STATES PATENTS 2,450,509 Glock Oct. 5, 1948 2,606,866 Neish Aug. 12, 1952 2,775,535 Poole Dec. 25, 1956 2,931,759 Hill Apr. 5, 1960 2,974,091 Neish Mar. 7, 1961 3,081,238 Gurry Mar. 12, 1963 

1. A METHOD OF TREATING TIN PLATE SO AS TO PROVIDE IMPROVED LACQUER ADHERENT SURFACE CHARACTERISTICS WHICH COMPRISES IN COMBINATION THE STEPS OF MAKING THE TIN PLATE THE CATHODE WHILE IMMERSED IN A HOT AQUEROUS ALKALINE CLEANING ELECTROLYTE AND PASSING AN ELECTRIC CURRENT THROUGH SAID ELECTROLYTE AND TIN PLATE TO REMOVE SUBSTANTIALLY ALL SURFACE OXIDES AND OTHER CONTAMINATION THEREON WITHOUT REMOVING TIN, WASHING SAID TIN PLATE WITH WATER AFTER IT IS WITHDRAWIN FROM SAID ALKALINE CLEANING ELECTROLYTE TO RINSE AWAY MOST OF SAID ALKALINE CLEANING ELECTROLYTE REMAINING ON THE SURFACE THEREOF, THEREAFTER APPLYING TO SAID TIN PLATE AFTER WASHING WITH WATER AND BEFORE SUBJECTING TO ELECTROLYTIC TREATMENT WITH A CHROMINUM CONTAINING SOLUTION AN ACIT CONDITIONING SOLUTION COMPRISING A HOT DILUTE AQUEOUS ACID SOLUTION HAVING A PH BETWEEN ABOUT PH 4 AND 6, AND DIRECTLY PASSIVATING THE ACID CONDITIONED TIN PLATE SURFACE BY MAKING SAID TIN PLATE THE CATHODE WHILE IMMERSING IN A HOT AQUEOUS HEXAVALENT CHROMIUM CONTAINING ELECTROLYTE HAVING A PH BETWEEN ABOUT PH 4 AND 6 WHILE PASSING AN ELECTRIC CURRENT. THROUGH SAID CHROMIUM CONTAINING ELECTROLYTE AND TIN PLATE TO PROVIDE A PASSIVE NONOXIDING SURFACE ON SAID TIN PLATE, WHEREBY A TIN PLATE HAVING SUBSTANTIALLY IMPROVED LACQUER ADHERENT SURFACE CHARACTERISTICS IS PROVIDED. 